Magnetic random access memory (MRAM) has several advantages over other types of non-volatile memory, such as Flash memory. For example, MRAM usually consumes less power and is faster to read and write data. MRAM also offers a promising alternative to some forms of volatile memory, such as dynamic random access memory (DRAM).
A conventional MRAM cell typically includes a magnetoresistive element which has a pair of ferromagnetic layers separated by a non-magnetic layer. One of the ferromagnetic layers has a relatively low coercivity and the other has a relatively high coercivity. The low- and high-coercivity layers are usually referred to as “free” and “pinned” layers respectively.
To store data in the cell, an external magnetic field is applied which orientates the magnetisation of the free layer. After the magnetic field is removed, the orientation of the magnetisation is retained.
To read data from the cell, a current is driven through the element. The magnetoresistance of the element is relatively high if the magnetisations of the layers are arranged in anti-parallel (AP) and is relatively low if the magnetisations of the layers are arranged in parallel (P). Thus, the state of the cell can be determined by measuring the magnetoresistance of the element.
The external magnetic field is generated by passing a current through at least one conductive line running close to the element. However, this arrangement suffers the problem that as the size of the cell decreases, the magnetic field required to switch the free layer increases and so power consumption also increases.
An alternative to applying an external magnetic field is to employ spin transfer switching, as proposed in “Current-driven Excitation of Magnetic Multilayers” by J. C. Slonczewski, p. 9353, Phys. Rev. B, Vol. 54 (1996), and reference is made to “Highly scalable MRAM using field assisted current induced switching” by W. C. Jeong et al., p. 184, 2005 Symposium on VLSI Technology Digest of Technical Papers.
In spin transfer switching, a current is driven through the magnetic element perpendicular to the layer interfaces. This causes spin-polarised electrons to be injected into the free layer either by electrons flowing through the pinned layer (when current is driven from the free layer to the pinned layer) or by electrons scattering from the pinned layer (when current is driven from the pinned layer to the free layer). When spin polarised electrons are injected into the free layer, they interact with the free layer and transfer a portion of their spin angular momentum to the magnetic moment of the free layer. If the spin-polarised current is sufficiently large, then this can cause the magnetisation of the free layer to switch.
A drawback, however, of spin transfer switching is that a high current density (e.g. of the order of 108 Acm−2) is needed to trigger the reversal process.
The current may be reduced by applying a dc pre-charging current before applying a switching current pulse, as described in “Precharging strategy to accelerate spin-transfer switching below the nanosecond” by T. Devolder et al., Applied Physics Letters, volume 86, page 062505 (2005). Although the power consumption of the switching current pulse is reduced, the overall power consumption (i.e. including the power consumption of the pre-charging current) is still quite large.
However, the current may be reduced by applying a short (e.g. <5 ns) external magnetic field pulse along a magnetic hard axis of the free layer immediately prior to or simultaneously with applying a switching current pulse so as to cause precessional switching, as described in “Micromagnetic simulation of spin transfer torque switching combined with precessional motion from a hard axis magnetic field” by K. Ito et al., Applied Physics Letters, volume 89, page 252509 (2006).
Although this technique can reduce the spin transfer current significantly, it involves applying an external magnetic field by passing a current through a line. This limits the potential for scalability and reducing power consumption.
The present invention seeks to ameliorate this problem.